Whereas cars and light goods vehicles usually have a hydraulic braking system supported by pedal force, and heavy goods vehicles have a pneumatic braking system supported by external force, wheeled vehicles such as e.g. construction machines and agricultural tractors, which are intended to be used off solid roads and paths, are equipped with an external force-supported hydraulic brake system. Because of the necessary high braking forces, the wheel brake cylinders of such wheeled vehicles have a relatively large displacement volume which can only be conveyed with an external force-supported braking system. Since construction machines and agricultural tractors normally use hydraulic working equipment or comprise a hydrostatic drive, an external force-supported hydraulic braking system can advantageously utilize the hydraulic pressure supply device which is present in any case.
In the present case, an external force-supported hydraulic braking system of a wheeled vehicle is assumed which has two brake circuits, a valve arrangement of an anti-lock braking system (ABS™ system), and a valve arrangement of a traction control system (TCS™ system). The brake circuits each have a brake valve which can be actuated by a brake pedal, by means of which a main brake line—from which several wheel brake lines branch off, each leading each to a wheel brake cylinder of the wheel brake—can be connected to a pressure-carrying supply line for actuation of the wheel brakes and to a pressureless return line for release of the wheel brakes.
The valve arrangement of the ABS system comprises several inlet valves, each with a check valve connected in parallel and opening in the direction of the respective main brake line, and several outlet valves. An inlet valve with a check valve and an outlet valve leading from the respective wheel brake cylinder to a return line are arranged at each wheel brake in the corresponding wheel brake line. In the non-actuated state, the inlet valves are opened and the outlet valves closed. If, during a braking process, by evaluation of rotation speed sensors assigned to the vehicle wheels, a locked vehicle wheel or one at risk of locking is established, the inlet valve of the assigned wheel brake cylinder is closed and the corresponding outlet valve opened, whereby the wheel brake of the vehicle wheel concerned is released. If, for example after releasing the brake pedal, a higher pressure is present in one of the wheel brake cylinders than in the assigned main brake line, this pressure is dissipated, when the inlet and outlet valves are closed, via the check valve connected in parallel to the inlet valve.
The TCS system also uses the inlet and outlet valves of the ABS system. If, when the wheeled vehicle moves away or accelerates, the rotation speed sensors arranged on the vehicle wheels detect a slipping drive wheel, a pressure-carrying pressure line is connected directly via the valve arrangement of the TCS system, i.e. bypassing the brake valves, to the main brake lines of the two brake circuits, and at the same time the return lines, which are connected to the main brake lines via the brake valves when the brake pedal is not actuated, are shut off. Immediately before this, the inlet valves of the brake cylinders of the non-driven vehicle wheels and the non-slipping drive wheels are closed, so that only the wheel brake of the slipping drive wheel is actuated. By braking the slipping drive wheel, the braking moment absorbed by the wheel brake there is active at the opposite drive wheel of the same drive axle via the differential to the same level as the drive moment, whereby the wheeled vehicle can move away or accelerate further. In comparison with a limited slip differential arranged on a drive axle, with which a traction loss on one drive wheel can also be prevented, a traction control system which acts via active brake intervention constitutes a relatively simple and significantly cheaper device.
DE 195 46 056 A1 describes an external force-supported hydraulic braking system of a motor vehicle with two brake circuits, an ABS system and a TCS system. The motor vehicle concerned for example has one drive axle and one non-driven vehicle axle. The wheel brake cylinders of the wheel brakes are distributed diagonally over the two brake circuits. For the TCS system, a pressure-carrying pressure line is provided which, bypassing the brake valves combined in the present case into a metering valve, leads from a pressure source with a branch to the wheel brake lines of the drive wheels. In a first embodiment, according to FIG. 1 there, the valve arrangement of the TCS system has a shut-off valve arranged before the branch in the pressure line and configured as a 2/2-way magnetic switching valve, and two check valves each arranged after the branch for infeed of the pressure medium. In a second embodiment, according to FIG. 2 there, the valve arrangement of the TCS system has two shut-off valves each arranged after the branch in the pressure line branches and configured as 2/2-way magnetic switching valves, for infeed of the pressure medium. Also, both embodiments of the valve arrangement of the TCS system each have, in each brake circuit, an isolating valve arranged between the branch of the main brake line and the opening of the respective pressure line branch into the wheel brake line and configured as a 2/2-way magnetic switching valve, to prevent the outflow of pressure medium.
EP 1 013 528 B1 describes various embodiments of a hydraulic brake system of a wheeled vehicle with two brake circuits and an actuation of wheel brakes independently of the brake valves, in which the actuation valves of the valve arrangement are arranged between the respective brake valve and the pressure supply device. The motor vehicle concerned has for example two vehicle axles. The wheel brake cylinders of the wheel brakes are divided per axle over the two brake circuits. Due to the valve arrangement, only the wheel brakes of one brake circuit or one vehicle axle can be actuated.